1. Field of the Invention
This invention relates to power supply circuits for multiple parallel electrical loads and, more particularly, to a ballast circuit having a current-balancing transformer for supplying electrical power to multiple parallel negative and/or non-linear impedance loads, such as gas discharge lamps.
2. Description of the Prior Art
A gas discharge lamp, e.g., a fluorescent lamp, is an electrical device which exhibits certain special electrical characteristics; among them, a negative impedance characteristic, which means that once the arc has been struck, increased current through the discharge medium within the lamp results in decreased voltage drop between the lamp electrodes; a positive impedance characteristic, which means that during normal operation at high frequency (frequencies greater than approximately 300 Hz) the lamp appears essentially as a resistive device throughout the high frequency cycle; and a non-linear impedance, which means that during the application of low frequency voltage the impedance changes during the cycle. A fluorescent lamp powered from a high frequency inverter (say 20 kHz) operated from an unfiltered rectified 60 Hz source exhibits all three impedance characteristics simultaneously. Because of these characteristics, it is necessary to provide means for current limitation in the ballast circuit. If current limitation means are not provided, lamp failure or ballast burnout generally results. An efficient fluorescent lamp ballast can be inductive, capacitive or dynamically controlled by a high frequency inverter. The most typical fluorescent ballast is an inductor which exhibits an inductive impedance. Additionally, because of the negative impedance characteristic, parallel operation of gas discharge lamps is generally precluded even though it provides certain desirable features, because one lamp will divert all the current. Furthermore, when parallel operation is attempted, the arc in one lamp is generally struck first, and this eventually carries all of the current supplied to the parallel lamp combination preventing starting of other lamps. Thus, conventional parallel operation results in only one lamp of a parallel-connected set being started. All the rest stay dark. Clearly, such a mode of operation is not tolerable. Accordingly, series operation of gas discharge lamps has been considered to be the only viable mode of operation. However, series operation of gas discharge lamps operated at high frequency (20 kilohertz and above) may produce the undesirable result of capacitively coupled leakage currents through the glass lamp envelope. This phenomenon is more significant in series-connected lamps, because larger voltage drops can occur along the lamp string than along a single lamp or parallel combination of lamps. Ballast circuit designs also incorporate a means for lamp starting. Therefore, it should be appreciated that the discussion above, and herein generally, relates to both starting lamps and driving lamps which have already been started.
Further discussion of lamp ballast circuit requirements is recited in U.S. patent application Ser. No. 292,324 filed by Victor David Roberts on Aug. 12, 1981, and assigned to the present assignee, now abandoned. In the above-identified patent application, a solution is presented to the current-sharing problem for more than two parallel negative and/or non-linear impedance loads by supplying power to each of a plurality of parallel discharge lamps from separate windings wrapped upon separate core legs of a multi-legged supply transformer. Power is supplied to a primary winding wrapped upon a first leg of the transformer core, and identical windings wrapped upon parallel secondary core legs provide output to each of the plurality of parallel discharge lamps. This construction provides flux sharing within the transformer core between the secondary core legs with full volt-second core requirements on each secondary leg. Therefore, the lamp loads are effectively connected in series with each other.
In FIG. 1, a prior art ballast circuit configuration is shown in which two parallel gas discharge lamps 38, 40 are connected to separate coils 26, 28 wound upon a magnetic core 24. A single main ballast inductor 20 is used to supply current to windings disposed upon the core. This configuration will tolerate only small lamp-to-lamp voltage differences and is not readily extended to a ballast circuit for driving more than two lamps due to the fact that a third winding placed upon the core 24 would result in an unequal flux sharing, since in order for the fluxes to balance, one winding must be creating flux which opposes the flux generated by the other two windings. In order to provide flexibility and practicality in the design of gas discharge lamp systems, a ballast circuit for driving more than two parallel-connected lamps is required.